The present invention relates to an electric cable, particularly of the high or even very high voltage variety having an insulative envelope made of a cross-linking material.
The German printed patent application DAS 1,794,028 discloses a method for cross-linking organic polymers by grafting of alcoxy-silane compounds on a polymer and curing the resulting graft polymer in the presence of moisture to obtain the cross-linking. For example, polyethylene is mixed with an alcoxy compound (silane) having the structure and R Si Y.sub.3 under conditions, in which free radicals of the polymerisate is formed. R represents a vinyl group or an alpha-methacryl-oxypropyl group and 1/3 is an alcoxy group with less than six carbon atoms. During or after mixing a silanol condensation catalyst can be added. Other additives may include organic peroxides which furnish free radicals. The material can be mechanically worked, for example, in an extruder, in which the mixture is homogenized, while the grafting takes place as the result of the heating process that takes place in the extrusion press.
The mixtures in accordance with the aforementioned procedure cross-link in the presence of moisture. Thus, granulation and storage of the grafted (but not yet cross-linked) material presents a significant problem and cross-linking may well begin prematurely which renders subsequent working difficult or even impossible.
It was found that the problem posed by premature cross-linking was compounded when the grafted material is to be used to make an electrically conductive composition by adding, for example, soot, i.e. carbon black, to the polymer. Such carbon black is very hygroscopic and inherently contains significant amounts of moisture. Even if subjected to drying all moisture cannot be removed from carbon black. It follows that a conductive mixture which cross-links in the presence of moisture (e.g. siloxan bridges or links) are hardly usable for the making of a cable, because the time period between completing the mixture and completing a cable jacket to the extent that it can cross-link is too long, so that premature cross-linking is more or less inevitable.
Electrical cable and conductors particularly when used for high or even very high voltages are usually provided with a surface finishing envelope or coating, which smoothes the surface of the conductor or envelopes a bundle of stranded conductor filaments in a thin jacket with smooth, round overall surface. Any roughness would increase local field strength. Such surface finishing coating is particularly advantageous, when made of cross-linking material. In connection therewith it is customary to cross-link this surface finish coating as well as the insulation envelope around the coating in that the layers are extruded and are passed through a heating station, because cross-linking is often carried out at elevated temperatures.